Method for producing aliphatic polyester having increased molecular weight

ABSTRACT

The problem to be solved by the present invention is to provide a method for continuously and effectively producing an aliphatic polyester having an increased molecular weight which prevents evaporation of diisocyanate at the step of mixing the diisocyanate with the aliphatic polyester prepolymer in a molten state at a temperature not lower than the melting point of the prepolymer and enables uniform mixing of the diisocyanate with the aliphatic polyester prepolymer. In order to solve this problem, the present invention provides a method for producing an aliphatic polyester having an increased molecular weight comprising the steps of:
         (i) quantitatively injecting a diisocyanate into an aliphatic polyester prepolymer which has a number average molecular weight of 5000 or higher and has terminal hydroxyl groups and in which at least one acid component is a succinic acid compound, in a molten state at a temperature not lower than the melting point thereof, wherein   the amount of diisocyanate is equivalent to between one tenth and two times the amount of the hydroxyl groups in the aliphatic polyester prepolymer,   (ii) quantitatively and continuously mixing the diisocyanate-containing aliphatic polyester prepolymer obtained at step (i) by a static mixer, discharging same from the static mixer, and then supplying same to a coupling reaction tank, and   (iii) reacting the aliphatic polyester prepolymer with the diisocyanate in the coupling reaction tank.

TECHNICAL FIELD

The present invention relates to a method for continuously andeffectively producing an aliphatic polyester having an increasedmolecular weight. The obtained polyester having an increased molecularweight is a high-quality resin.

BACKGROUND ART

Conventionally, the high molecular weight polyesters used in films,sheets, fibers, and other molded articles were mostly aromaticpolyesters such as polyethylene terephthalate and polybutyleneterephthalate.

Aliphatic polyesters generally regarded as being biodegradable haveattracted attention in recent years from the point of environmentalprotection and the like. As a method for producing the aliphaticpolyesters, a method comprising directly esterifying an aliphaticdicarboxylic acid with an aliphatic diol or a method comprisingconducting transesterification of an alkyl ester of aliphaticdicarboxylic acid or an anhydride thereof with an aliphatic diol toobtain a glycol ester or a low polymer thereof and then stirring thesame by heat under high vacuum for polycondensation has been known.

Polymerization proceeds by removing a released component outside thesystem in the polycondensation reaction as above. It is generallyconducted by distilling the released component from the systemconditioned at a high temperature and under high vacuum. For example,Patent publication 1 discloses a method for producing an aliphaticpolyester having an increased molecular weight comprising esterifying analiphatic dicarboxylic acid with a glycol component and conducting areaction to remove glycol from the formed polyester diol in the presenceof a catalyst at a temperature of 180-230° C. under high vacuum at0.005-0.1 mmHg.

However, it is not sufficient to put the system under high vacuum, butit is necessary to make the surface area of the reactants forpolycondensation sufficiently large and effectively renewed. This isbecause the released component present on the surface of the reactantscan be removed more easily. In particular, as the viscosity of thereactants is increased and it is difficult for the released component todiffuse from the reactants in the latter half of the polycondensationreaction, it is necessary to make the surface area of the reactantslarger by mechanical stirring so as to effectively renew the surface. Asthe desired surface area and effective surface renewal cannot beachieved in using a conventional polymerization apparatus equipped withconventional stirring blades, problems where the reaction does notproceed sufficiently and an aliphatic polyester having an increasedmolecular weight cannot be obtained occur. Formation of a film or sheetusing a polybutylene succinate having a low molecular weight isdifficult.

In addition, as there is a limit to the direct polymerization describedabove, Patent publication 2 proposes preparation of an aliphaticpolyester having an increased molecular weight by adding a diisocyanatehaving an isocyanate group in an amount one tenth to two timesequivalent to the amount of the hydroxyl groups to an aliphaticpolyester prepolymer substantially having terminal hydroxyl groups in amolten state at a temperature not lower than the melting temperaturethereof. It has been known that the aliphatic polyester having anincreased molecular weight obtained by this method has a weight averagemolecular weight (Mw) of 200,000 or higher that cannot be achieved by aconventional direct polymerization method and that formability andphysical properties of films thereof are excellent. However, by thismethod, reduction of accuracy in controlling molecular weight andformation of gelation and fish eyes sometimes occurred when adding theisocyanate to a polymerization tank as in the conventional method.

PRIOR ART REFERENCE Patent Publications

-   Patent publication 1: Japanese Patent Laid-Open No. 5-310898-   Patent publication 2: Japanese Patent No. 2825969

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The inventors of this application have studied the reduction of accuracyin controlling molecular weight and formation of gelation and fish eyesand as a result, they found that when an isocyanate is added to apolymerization tank at a high temperature not lower than the meltingpoint of the aliphatic polyester prepolymer so as to increase themolecular weight, the isocyanate evaporates due to the high temperatureand not all of the amount of isocyanate added is used for the reactionand that the evaporated isocyanate is adhered to the inner wall of thereaction tank vapor-phase portion.

Accordingly, the object of the present invention is to provide a methodfor continuously and effectively producing an aliphatic polyester havingan increased molecular weight which prevents evaporation of diisocyanateat the step of mixing the diisocyanate with the aliphatic polyesterprepolymer in a molten state at a temperature not lower than the meltingpoint of the prepolymer and enables uniform mixing of the diisocyanatewith the aliphatic polyester prepolymer.

Means for Solving the Problems

The inventors of this application keenly studied to achieve the aboveobject, and as a result, found that a high-quality aliphatic polyesterhaving an increased molecular weight in which gelation or formation offish eyes are reduced can be obtained by quantitatively injecting adiisocyanate into an aliphatic polyester prepolymer in a molten state ata temperature not lower than the melting point thereof, sufficientlystirring and uniformly mixing same, and then subjecting same to acoupling reaction.

Namely, the present invention is:

(1) a method for producing an aliphatic polyester having an increasedmolecular weight comprising the steps of:

(i) quantitatively injecting a diisocyanate into an aliphatic polyesterprepolymer which has a number average molecular weight of 5000 or higherand has terminal hydroxyl groups and in which at least one acidcomponent is a succinic acid compound, in a molten state at atemperature not lower than the melting point thereof, wherein

the amount of the diisocyanate is equivalent to between one tenth andtwo times the amount of the hydroxyl groups in the aliphatic polyesterprepolymer,

(ii) quantitatively and continuously mixing the diisocyanate-containingaliphatic polyester prepolymer obtained at step (i) by a static mixer,discharging same from the static mixer, and then supplying same to acoupling reaction tank, and

(iii) reacting the aliphatic polyester prepolymer with the diisocyanatein the coupling reaction tank;

(2) a method for producing an aliphatic polyester having an increasedmolecular weight according to (1),

wherein a mixing tank is further provided between the static mixer andthe coupling reaction tank and the aliphatic polyester prepolymer of(ii) discharged from the static mixer is introduced into the mixing tankand is further discharged from the mixing tank under stirring to besupplied into the coupling reaction tank;

(3) a method for producing an aliphatic polyester having an increasedmolecular weight according to (2),

wherein the mixing tank of (ii) has an inlet of thediisocyanate-containing aliphatic polyester prepolymer in its upperportion, an outlet for discharging a mixture of the aliphatic polyesterprepolymer with the diisocyanate in its bottom portion, and at least onepartitioning plate and stirring blade;

(4) a method for producing an aliphatic polyester having an increasedmolecular weight according to (3),

wherein said at least one partitioning plate and stirring blade arespaced one above the other;

(5) a method for producing an aliphatic polyester having an increasedmolecular weight according to (3) or (4),

wherein the stirring blade is a multistage blade;

(6) a method for producing an aliphatic polyester having an increasedmolecular weight according to any one of (1)-(5),

wherein the diisocyanate is injected by a non-pulsating pump at step (i)above;

(7) a method for producing an aliphatic polyester having an increasedmolecular weight according to any one of (1)-(6),

wherein the reaction at step (iii) above is conducted under stirring;

(8) a method for producing an aliphatic polyester having an increasedmolecular weight according to (7),

wherein the reaction at step (iii) above is conducted under stirring bya helical ribbon blade or a twisted lattice-shape blade;

(9) a method for producing an aliphatic polyester having an increasedmolecular weight according to any one of (1)-(6),

wherein the reaction at step (iii) above is conducted without stirring;

(10) a method for producing an aliphatic polyester having an increasedmolecular weight according to any one of (1)-(9),

wherein at least one polyalcohol component in the aliphatic polyester isethylene glycol; and

(11) a method for producing an aliphatic polyester having an increasedmolecular weight according to any one of (1)-(9),

wherein at least one polyalcohol component in the aliphatic polyester is1,4-butanediol.

Effect of the Invention

According to the method of the present invention, a high-qualityaliphatic polyester having an increased molecular weight with lessoccurrence of gelation and less formation of fish eyes can becontinuously produced in increasing the molecular weight of an aliphaticpolyester. In addition, according to the production method of thepresent invention, accuracy in controlling the melt-flow rate (MFR) ofthe obtained aliphatic polyester having an increased molecular weight ishigh.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic drawing of the production method of the presentinvention.

FIG. 2 is a schematic drawing of the production method corresponding toa comparative example.

FIG. 3 is an embodiment of mixing tank 7 used in the present invention.

MODE FOR CARRYING OUT THE INVENTION

The present invention relates to a method for continuously and stablyproducing an aliphatic polyester having an increased molecular weight onan industrial scale comprising injecting a diisocyanate into analiphatic polyester prepolymer which has a number average molecularweight of 5000 or higher and has terminal hydroxyl groups and in whichat least one acid component is a succinic acid compound, in a moltenstate at a temperature not lower than the melting point thereof, wherein

the diisocyanate comprises isocyanate groups in an amount equivalent tobetween one tenth and two times the amount of the hydroxyl groups in thealiphatic polyester prepolymer.

The succinic acid compound used in the present invention is succinicacid or a derivative thereof (such as diesters, monoesters, andanhydride thereof). Specific examples thereof include succinic acid;succinic acid esters such as dimethyl succinate and diethyl succinate;and succinic anhydride. Among them, succinic acid, dimethyl succinate,and succinic anhydride are preferable. Succinic acid or derivativesthereof may be used alone or in combination of two or more thereof.

Dicarboxylic acid compounds other than the succinic acid compound may beused as part of the acid components above. Specific examples thereofinclude dicarboxylic acids having a linear or branched alkylene groupsuch as adipic acid, suberic acid, sebacic acid, azelaic acid, decanedicarboxylic acid, dodecane dicarboxylic acid, octadecane dicarboxylicacid, and dimeric acid; esters of the dicarboxylic acids such asdimethyl adipate and dimethyl malonate; acid anhydrides such as maleicanhydride, itaconic anhydride, and adipic anhydride; and oxycarboxylicacids such as malic acid, tartaric acid, and citric acid. Among them,adipic acid or an adipic acid derivative such as dimethyl adipate ispreferable.

The amount of the dicarboxylic acid component other than the succinicacid compound above is about 0-35 mol %, preferably about 0-25 mol %,with respect to the total amount of dicarboxylic acids as the acidcomponents.

Glycol is used as a polyalcohol for producing the aliphatic polyesterused in the production method of the present invention. Examples thereofinclude aliphatic glycols having a linear or branched alkylene groupsuch as ethylene glycol, 1,2-propanediol, 1,3-propanediol,1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol,1,5-pentanediol, 1,2-pentanediol, 2,4-pentanediol, 1,6-hexanediol,1,2-hexanediol, neopentyl glycol, and 2,2-diethyl-1,3-propanediol;epoxides corresponding to 1,2-glycol; alcohols having a valency of threeor higher such as trimethylolpropane; and diepoxides. Among them,ethylene glycol and 1,4-butanediol are preferable. 1,4-Butanediol ismore preferable.

The amount of the glycol component used is different, depending on thephysical properties of the desired polyester, but in general, it is1.02-1.5 mole, preferably 1.03-1.2 mole, with respect to one mole of thedicarboxylic acid component. If it is less than 1.02 mole, the contentof the aliphatic polyester prepolymer having terminal hydroxyl groupsbecomes small.

In general, an aliphatic polyester is produced in the presence of acatalyst. Catalysts may be used alone or in combination of two or morethereof. A wide range of catalysts used in transesterification reactioncan be used. Examples of the catalysts include protonic acids such assulfuric acid, p-toluenesulfonic acid, and phosphoric acid andderivatives thereof; metal compounds comprising a metal such as Li, Mg,Ca, Ba, La, Ce, Ti, Zr, Hf, V, Mn, Fe, Co, Ir, Ni, Zn, Ge, and Sn (forexample, organic metal compounds comprising the metal such as organicacid salts, metal alkoxides and metal complexes (acetylacetonate and thelike) and inorganic metal compounds comprising the metal such as metaloxides, metal hydroxides, carbonates, phosphates, sulfates, nitrates andchlorides.). Among these metal compound catalysts, titanium compounds,especially, organic titanium compounds such as titanium alkoxidesincluding titanium tetraethoxide titanium tetraisopropoxide, andtitanium tetrabutoxide are preferable. The amount of these metalcompound catalysts is about 0.005-1 mole, preferably about 0.01-0.3mole, with respect to 100 moles of the total amount of the acidcomponents.

In the method of the present invention, an organic or inorganicphosphorus compound may be used as a catalyst with the metal compoundcatalyst above (such as an organic titanium compound). A polymer havingan increased molecular weight can be obtained in a short polymerizationtime by using the metal compound above in combination with the organicor inorganic phosphorus compound.

The organic or inorganic phosphorus compound is exemplified as below.

(a) Phosphoric acid and organic esters thereof: commercially availableproducts thereof include phosphoric acid, alkyl or aryl acidicphosphates (in which the alkyl or aryl group is methyl, isopropyl,butyl, octyl, phenyl, and naphthyl group), and the like.

(b) Phosphonic acid and organic esters thereof: commercially availableproducts thereof include methyl phosphonate, ethyl phosphonate, arylphosphonates such as phenyl phosphonate and naphthyl phosphonate,dibutyl butyl phosphonate, and the like. Substituents such as alkylgroups (C1-4 alkyl groups and the like such as methyl group), halogenatoms (fluorine atom, chlorine atom, and the like), alkoxy groups (C1-4alkoxy groups and the like such as methoxy group), nitro group, and thelike may be bonded to the aromatic rings of the aryl phosphonates above.

(c) Phosphorous acid and organic esters thereof: examples thereofinclude dibutyl hydrogen phosphite, triphenyl phosphite, diphenylisodecyl phosphite, and tris isodecyl phosphite.

The amount of the organic or inorganic phosphorus compound in using themetal compound catalyst in combination with the organic or inorganicphosphorus compound as catalysts is 1-100 moles, preferably 5-33 moles,with respect to 100 moles of the metal compound catalyst (such as anorganic titanium compound).

The type of diisocyanate used in the present invention is notparticularly limited, but a commercially available product per se can beused.

Examples thereof include 2,4-trilene diisocyanate, a mixture of2,4-trilene diisocyanate and 2,6-trilene diisocyanate, diphenylmethanediisocyanate, P,P′-diphenyl diisocyanate, 1,6-naphthylene diisocyanate,xylylene diisocyanate, hydrogenated xylylene diisocyanate, isophoronediisocyanate, and hexamethylene diisocyanate.

Next, embodiments of the present invention will be explained withreference to FIG. 1. FIG. 1 is an example of the scheme of theproduction method of the present invention.

In the specification of this application, the descriptions, i.e.,weighing hopper 1 for supplying the acid component comprising thesuccinic acid compound to esterification tank 3, glycol storage tank 2from which the glycol component is supplied to esterification tank 3,esterification tank 3 in which the acid component and the glycolcomponent are mixed and esterified, transesterification reaction tank 4in which glycol is removed by transesterification reaction from thealiphatic polyester prepolymer formed in esterification tank 3,diisocyanate storage tank 5, static mixer 6 for mixing the aliphaticpolyester prepolymer with the diisocyanate, mixing tank 7 optionallyprovided in which the aliphatic polyester prepolymer is further mixedwith the diisocyanate, coupling reaction tank 8 for obtaining thealiphatic polyester having an increased molecular weight in which themixed aliphatic polyester prepolymer and diisocyanate are reacted, andpelletizer 9 for pelletizing the aliphatic polyester having an increasedmolecular weight obtained as a final product are used.

Glycol storage tank 2 and esterification tank 3 are connected by pipe 10a. Esterification tank 3 and transesterification reaction tank 4 areconnected by pipe 10 b. Transesterification reaction tank 4 and staticmixer 6 are connected by pipe 10 c Diisocyanate storage tank 5 isconnected to pipe 10 c via pipe 10 d. Static mixer 6 and mixing tank 7are connected by pipe 10 e. Mixing tank 7 and coupling reaction tank 8are connected by pipe 10 f. Coupling reaction tank 8 is connected topelletizer 9 by pipe 10 g.

The acid component comprising the succinic acid compound is suppliedfrom weighing hopper 1 to esterification tank 3. When an acid componentother than the succinic acid compound is used, a mixture previouslyprepared with the succinic acid compound may be supplied from weighinghopper 1 to esterification tank 3 or another weighing hopper may be usedto supply the other acid component to esterification tank 3 separatelyfrom the succinic acid compound.

A catalyst is added from a tank (not shown) for supplying the catalystto esterification tank 3. The catalyst used at each step may be added toesterification tank 3 at the same time or a catalyst needed intransesterification reaction tank 4 may be added.

Glycol is supplied through pipe 10 a to esterification tank 3. Pipe 10 ais provided with pump 11 a and a weighing apparatus for weighing theamount of glycol supplied, so that the supply by the pump can beautomatically stopped. The pump used herein includes centrifugal pumps,turbine pumps, propeller pumps, and the like.

In addition, the temperature of pipe 10 a is set to a temperature notlower than the melting point of glycol so as to prevent crystallizationof the glycol. The temperature is different, depending on the kind ofglycol, but in general, it is preferably set to 30 to 50° C.

Ingredients are preferably reacted under stirring in esterification tank3 for promoting the reaction efficiently. Esterification tank 3 ispreferably provided with a stirring blade. For example, a verticalstirring tank equipped with a stirring blade having a vertical rotationshaft (such as a paddle blade and a turbine blade) is used.

After the predetermined amounts of the acid component, the glycolcomponent, and the catalyst are supplied to esterification tank 3,esterification reaction is conducted under a nitrogen gas atmosphere. Ingeneral, the reaction temperature is from 140 to 250° C., preferablyfrom 145 to 245° C. If the temperature is lower than 140° C., thereaction rate is slow and is not practical. If the temperature is higherthan 250° C., the formed polymer may be thermally decomposed. Ingeneral, the reaction pressure is normal pressure, but the pressure inthe system may be reduced in the latter half of the reaction, forexample, to from 5 to 100 mmHg (from 665 to 13300 Pa) so as to shortenthe reaction time. The reaction time is not particularly limited, but isgenerally from 6 to 12 hours.

The aliphatic polyester prepolymer obtained in esterification tank 3 hasa number average molecular weight of from about 500 to 5,000.

The aliphatic polyester prepolymer obtained in esterification tank 3 israpidly delivered to transesterification reaction tank 4 through pipe 10b. A catalyst is added as necessary for conducting thetransesterification reaction. At this step, the molecular weight of theformed aliphatic polyester prepolymer is increased mainly bytransesterification reaction (condensation reaction by removing glycol)of the aliphatic polyester prepolymers. When the number averagemolecular weight becomes 5,000 or higher, an aliphatic polyesterprepolymer having terminal hydroxyl groups is formed.

The temperature in the transesterification reaction is from 200 to 250°C., preferably from 210 to 240° C. If the polymerization temperature istoo low, the reaction time becomes longer and the production efficiencyis lowered. If the polymerization temperature is too high, the polymeris colored and decomposed products are easily formed. In order topromote the condensation reaction by removing glycol, the pressure inthe system should be reduced. The pressure finally reached at the timeof completion of reaction is from 0.1 to 5.0 mmHg (from 13 to 666 Pa)and the reaction time is about 5 to 10 hours.

Although the esterification reaction and the transesterificationreaction in the present invention are bulk polymerization without usingsolvents, solvents may be used as necessary.

Polymerization apparatuses generally used in polyester prepolymerproduction such as esterification tank 3 above and couplingpolymerization apparatuses can be used as transesterification tank 4.Examples of these polymerization apparatuses include vertical stirringtanks equipped with a stirring blade having a vertical rotation shaft(such as a double helical ribbon blade and a twisted lattice-shapeblade).

The aliphatic polyester prepolymer having a number average molecularweight of 5,000 or higher and having terminal hydroxyl groups isdelivered to static mixer 6 through pipe 10 c. The number averagemolecular weight of the aliphatic polyester prepolymer generallyobtained by transesterification reaction is from 5000 to 30,000,preferably from 10,000 to 20,000.

As the viscosity of the aliphatic polyester prepolymer is high, it isnecessary to raise the temperature to turn it into a molten state inmixing the aliphatic polyester prepolymer with the diisocyanate.However, if the aliphatic polyester prepolymer in a molten state issimply mixed with the diisocyanate, the diisocyanate evaporates to moveto the head space of the mixing tank and cannot be mixed uniformly atthe predetermined ratio. In addition, the diisocyanate adheres to theinner wall surface of the mixing tank vapor-phase portion, and does notcontribute to the reaction, so, a prepolymer having a sufficientlyincreased molecular weight cannot be obtained.

For that reason, evaporation of the diisocyanate can be prevented byusing a pump and quantitatively injecting the diisocyanate withoutgenerating vapor-phase diisocyanate directly into the aliphaticpolyester prepolymer through an inlet provided in pipe 10 c whichenables injection of the diisocyanate.

As another embodiment, static mixer 6 is provided with an inlet whichenables injection of the diisocyanate and the diisocyanate may beinjected from the inlet (not shown). The inlet is preferably provided atthe upstream portion (on the side of pipe 10 c) from the center of thestatic mixer.

Pipe 10 c is provided with pump 11 b. This pump is provided fordelivering the high-temperature aliphatic polyester prepolymer having ahigh viscosity and needs to be a quantitative pump which can deliver theprepolymer quantitatively and continuously through pipe 10 c. Thequantitative pump used herein is preferably a volume pump, a rotatingpump that can be used for the prepolymer having a high viscosity.Specific examples of the rotating pump include gear pumps, screw pumps,and vane pumps. Among them, gear pumps which have high quantitativityand are the most suitable for delivering a fluid having a high viscosityare preferable.

In order to increase the quantitativity, the quantitative pump is drivenby a servomotor, whereby the flow rate can be controlled.

The delivery rate of the aliphatic polyester prepolymer through pipe 10c is affected by the scale of the apparatus, the diameter of pipe 10 cand the like, but generally is maintained to be constant by which theaverage residence time in the mixing tank at the following step becomesfrom 2 to 10 minutes. If the delivery rate is faster than the above, thediisocyanate may not be sufficiently mixed with the aliphatic polyesterprepolymer. If it is too slow, the reaction proceeds excessively toincrease the viscosity of the polymer, which cannot be easily dischargedfrom the mixing tank.

The rate is adjusted mainly by the rotational speed of pump (11 b).

The aliphatic polyester prepolymer in a molten state is delivered insuch a manner that it is filled in pipe 10 c. Namely, pipe 10 c isfilled with the aliphatic polyester prepolymer such that there are fewvoids in the pipe. The void ratio in pipe 10 c is less than 5%,preferably less than 2%, more preferably less than 1%. It is preferablethat the diisocyanate is injected into the aliphatic polyesterprepolymer without contacting the void portion (vapor phase) in pipe 10c. If the void ratio is large, the diisocyanate injected from pipe 10 devaporates and remains in the void space, whereby the diisocyanatecannot be efficiently mixed with the aliphatic polyester prepolymer atthe following step.

If static mixer 6 is provided with an inlet, through which thediisocyanate is injected, static mixer 6 is filled with the aliphaticpolyester prepolymer in a molten state in the same manner as describedabove. The void ratio in static mixture 6 is less than 5%, preferablyless than 2%, more preferably less than 1%. It is preferable that thediisocyanate is injected into the aliphatic polyester prepolymer withoutcontacting the void portion (vapor phase) in static mixer 6.

Diisocyanate storage tank 5 is connected to pipe 10 c or the inletprovided in static mixer 6 via pipe 10 d. Pipe 10 d is provided withquantitative pump 11 c, which enables a quantitative and continuoussupply of the diisocyanate to the aliphatic polyester prepolymer. Thequantitative pump used is preferably a non-pulsating pump. In this case,a reciprocating pump suitable for delivering a small amount of a fluidhaving a low viscosity is preferably used. Examples thereof includediaphragm pumps, piston pumps, and plunger pumps. It is preferable touse plunger pumps connected in line so as to stabilize the flow rate andincrease the accuracy. In order to further increase the quantitativity,the quantitative pump may be driven by a servomotor to control the flowrate.

The non-pulsating pump used herein means a pump comprising a pluralityof pumps in line whose total volume is constant, namely which can pumpflow volumes without pulsation. One is used in a wide range ofapplications.

The diisocyanate in an amount equivalent to one tenth to two times theamount of the hydroxyl groups of the aliphatic polyester prepolymer isquantitatively injected. A pump is used to pressurize the diisocyanatein pipe 10 d at from 0.5 to 1 MPa at room temperature.

When the isocyanate is injected through pipe 10 c, the injecteddiisocyanate-containing aliphatic polyester prepolymer is quantitativelyand continuously supplied from the inlet of static mixer 6 through pipe10 c. When the diisocyanate is injected into static mixer 6, injectingand delivering it into the static mixer occur at the same time.

The static mixer used in the present invention means a static mixingdevice which has no driving members, but mixes and disperses a fluid bymaking the fluid pass through the pipe and blades called mixing elementsarranged in line in the tube or flow path.

Static mixers used as static mixer 6 are grouped into two types, i.e.,spiral types and the stator type, depending on the shapes of mixingelements. The spiral type static mixer is preferably used because theinjected diisocyanate-containing aliphatic polyester prepolymer has ahigh viscosity. In order to obtain a sufficient mixed state, it ispreferable that the temperature be from 160 to 200° C. and the deliveryrate from 5.0 to 27 cm/sec when the aliphatic polyester prepolymer ispassed through the static mixer.

As the aliphatic polyester prepolymer is a fluid having a medium levelof viscosity such as several hundred poises, the number of elements ismust be 6 or more, preferably 12 or more, by which an excellent mixingstate can be obtained.

In the present invention, after the aliphatic polyester prepolymer ismixed by the static mixer, it is delivered directly to coupling reactiontank 8, where it can be reacted, but mixing tank 7 may be provided foranother mixing step.

When mixing tank 7 is used, the aliphatic polyester prepolymer mixed bythe static mixer is quantitatively and continuously provided from theinlet of mixing tank 7 through pipe 10 e.

A vertical mixer may be used as mixing tank 7, and has a stirring shaftequipped with stirring blades. Stirring blades are categorized intopropeller types, paddle types, turbine types, ribbon types, and specialpaddle types. These stirring members may be used in combinations of twoor more thereof.

Further, one embodiment of mixing tank 7 will be explained withreference to FIG. 3. Mixing tank 7 in FIG. 3 is provided with inlet 7 afor ingredients at its upper portion, outlet 7 b for the mixture at itsbottom portion, stirring shaft 7 c, partitioning plate 7 d, stirringblades 7 e and 7 f, and nitrogen inlet 7 g.

Inlet 7 a is connected to pipe 10 e. The injecteddiisocyanate-containing aliphatic polyester prepolymer is introducedfrom inlet 7 a as shown by a dashed arrow. The introduced ingredient isa fluid having a high viscosity. It is delivered to the bottom portionunder stirring. It is important to uniformly mix the ingredients so asto obtain a high-quality aliphatic polyester having an increasedmolecular weight. Therefore, partitioning plate 7 d is provided tomaintain an appropriate residence time so as to achieve sufficientstirring by the stirring blades. (Partitioning plate) 7 d is fixed to(stirring shaft) 7 c.

Stirring blades are provided above and below partitioning plate 7 d, 7 eand 7 f represent stirring blades in FIG. 3. Stirring blade 7 e is astirring blade arranged at an angle of 90° to the horizontal surface. 7f represents a stirring blade arranged at an angle of 45° or 135° to thehorizontal surface. One or more stirring blades can be provided. It ispreferable to use multistage blades as shown in FIG. 3 because efficientstirring can be realized.

It is important that the diisocyanate and the aliphatic polyesterprepolymer are delivered at a constant rate in static mixer 6 and mixingtank 7.

Although retention times of the introduced ingredients are different,depending on the scale and the like of static mixer 6 and mixing tank 7,in general, the introduced ingredients are retained in static mixer 6for from 5 to 30 seconds and mixing tank 7 for from 2 to 10 minutes andthen discharged.

Nitrogen introduced from nitrogen inlet 7 g is used for promotingdischarge of the polymerization product in the mixing tank. The pressurein the mixing tank is adjusted by nitrogen to from 0.15 to 0.5 MPa,preferably from 0.2 to 0.4 MPa. The stirring blade rotational speed isadjusted to from 50 to 200 rpm.

The mixture uniformly blended in static mixer 6 and mixing tank 7optionally provided is delivered to coupling reaction tank 8 through thepipe. A polymerization apparatus that may be used in transesterificationreaction tank 4 or a high viscosity polymerization apparatus may be usedas the coupling reaction tank. Examples of these polymerizationapparatuses include vertical stirring tanks equipped with a stirringblade having a vertical rotation shaft (such as a double helical ribbonblade and a twisted lattice-shape blade). Ingredients can be reactedwithout stirring in coupling reaction tank 8. In that case, the stirringblades in the stirring tank are not rotated or a reaction tank withoutstirring blades may be used.

The reaction temperature in coupling reaction tank 8 is from 130 to 210°C., preferably from 160 to 200° C. If the reaction temperature is toolow, as the prepolymer crystallizes and is not fluid, stirring cannot beachieved. If the temperature is too high, as the reaction rate becomestoo fast, sufficient stirring cannot be achieved. At this step, ingeneral, reaction is carried out for from 2 to 10 hours, preferably from3 to 8 hours under conditions at normal pressure and at a temperaturewithin the above range.

Nucleators, pigments, dyes, heat stabilizers, antioxidants, weatherresistant agents, lubricants, antistatic agents, fillers, reinforcingagents, flame retardants, plasticizers, other polymers, and the like maybe added after completion of the reaction, as necessary.

The aliphatic polyester having an increased molecular weight produced bythe above method is delivered to pelletizer 9 through pipe 10 g, whereit is cut into a desired shape (for example, pellets).

As the aliphatic polyester having an increased molecular weight has ahigh viscosity, pipe 10 g is equipped with pump 11 d. A pump similar topump 11 b may be used as pump 11 d.

The aliphatic polyesters having an increased molecular weight obtainedby the production method of the present invention can be formed intofilms, sheets, fibers, foam bodies, and other molded articles byconventional molding methods such as mold injection, hollow molding,extrusion, and the like. In addition, as the aliphatic polyesters havingan increased molecular weight obtained by the production method of thepresent invention are biodegradable, they are suitably used in garbagebags, agricultural films, cosmetic containers, detergents, and the like,fishing lines, fishnets, ropes, suture threads, food wrapping materials,medical containers, and the like.

EXAMPLES

The present invention will be explained in more detail below withreference to examples, but it is not limited thereto.

Molecular weight determination is carried out by the GPC analysis below.

Shodex GPC SYSTEM-11 (manufactured by SHOWA DENKO K.K.)

Eluent: CF₃COONa 5 mM/HFIP (hexafluoroisopropanol)

Sample column: HFIP-800P and HFIP-80M×two sets

Reference column: HFIP-800R×two sets

Polymer solution: 0.1 wt % HFIP sol., 200 μl

Column temperature: 40° C., flow rate: 1.0 ml/min, pressure: 30 kg/cm²

Detector: Shodex RI

Molecular weight standard: PMMA (Shodex STANFARD M-75)

MFR (melt flow rate) determination was carried out, in accordance withJIS-K-7210, at a temperature of 190° C. under the load of 2.16 kg.

Gel and/or FE (gelation and/or fish eyes) were determined by thefollowing steps. A film having a thickness of 30 μm was formed byinflation molding at a molding temperature of 180° C. The film was cutinto 50 cm×50 cm squares. Any Gel and/or FE (with a size of 0.2 mm orlarger) in a 30 cm×30 cm center space of the cut film were countedvisually. Ten Gel and/or FE portion with a size of from 0.2 to 0.5 mmwere evaluated as one Gel and/or FE portion with a size of 0.5 mm orlarger. Samples were evaluated to be assigned with any of five gradesdetermined based on the inequalities below.

Ns=(N×25)/T

T: film thickness (μm), N: number of Gel and/or FE portion with a sizeof 0.5 mm or larger

Evaluation 1: Ns<1

-   -   2: 1≦Ns<3    -   3: 3≦Ns<10    -   4: 10≦Ns<20    -   5: 20≦Ns

Regarding the determination of foreign substances, substances (such asblack spots) in 200 g of resin pellets were confirmed visually and wereevaluated to be assigned with any of grades A to E, depending on thesize and number thereof.

A: 10 or less foreign substances with a size of 0.1 mm or less, 5 orless foreign substances with a size of more than 0.1 and less than 0.5mm, 1 or less foreign substances with a size of 0.5 mm or more

B: 20 or less foreign substances with a size of 0.1 mm or less, 10 orless foreign substances with a size of more than 0.1 and less than 0.5mm, 2 or less substances with a size of 0.5 mm or more

C: 30 or less foreign substances with a size of 0.1 mm or less, 20 orless foreign substances with a size of more than 0.1 and less than 0.5mm, 3 or less foreign substances with a size of 0.5 mm or more

D: 31 or more foreign substances with a size of 0.1 mm or less, 50 orless foreign substances with a size of more than 0.1 and less than 0.5mm, 3 or less foreign substances with a size of 0.5 mm or more

E: 31 or more foreign substances with a size of 0.1 mm or less, 51 ormore foreign substances with a size of more than 0.1 and less than 0.5mm, 4 or more foreign substances with a size of 0.5 mm or more

Example 1

An aliphatic polyester was produced, in accordance with the productionflow chart shown in FIG. 1 (where mixing tank 7 is not included).

(Esterification Reaction)

All the ingredients, i.e., 3589 kg of 1,4-butanediol (39.8×10³ mol:glycol excess ratio of 104.4%) supplied from glycol storage tank 2 usinga centrifugal pump, 4500 kg of succinic acid (38.1×10³ mol) fromweighing hopper 1, and 810 g of titanium tetraisopropoxide from acatalyst supply tank were charged into a vertical stirring tank with ajacket equipped with paddle blades (esterification tank 3) under anitrogen atmosphere. The ingredients were stirred at a temperature of145 to 225° C. at normal pressure for carrying out the esterificationreaction. Water formed in the esterification reaction was distilled.When the amount of distilled water was greater than 1390 kg, thepressure in the tank was reduced to 60 mmHg (8000 Pa) and then wasmaintained for one hour to complete the esterification reaction. Thereactant solution was delivered to a vertical stirring tank with ajacket equipped with ribbon blades (transesterification reaction tank4). The delivered reactant solution was stirred at a temperature of 225to 240° C. The pressure in the tank was finally reduced to 2 mmHg (267Pa). After 10 hours, cooling was started. Reduction in pressure wasstopped when the temperature reached 190° C. and the inside of the tankwas put under a nitrogen atmosphere. Then, 3.28 kg of IRGANOX 1010(manufactured by BASF: hindered phenol-based antioxidant) was added, theingredients were cooled to 180° C., and 1.05 kg of phosphorous acid wasadded to conduct glycol removal reaction (transesterification reaction).The average molecular weight of the product determined by GPC analysiswas 10,600.

(Coupling Reaction Step)

The aliphatic polyester prepolymer obtained at the transesterificationreaction step above was delivered into static mixer 6 (manufactured byNORITAKE CO., LIMITED: 2 1/2(4)-N10ES-532-2 (WN)) at a constant flowrate (3250 kg/h) (total amount of 6500 kg) by a gear pump (pump 11 b) ata temperature of 180° C. Pipe 10 c was filled with the aliphaticpolyester prepolymer and there were no voids in the pipe. Hexamethylenediisocyanate was injected into the pipe at a flow rate of 35.15 kg/h(OH/NCO=1/0.68: total amount of 70.3 kg) by a non-pulsating pump (pump11 c) to be delivered to static mixer 6. The ingredients were agitatedand stirred in static mixer 6 and were delivered into coupling reactiontank 8. The coupling reaction by mixing was further conducted for 7hours at 180° C. under stirring with helical ribbon blades as long asthe blades could rotate.

After completion of the reaction, the resultant product was deliveredinto an extruder by a gear pump and pelletized by a pelletizer 9 toobtain pellets of an aliphatic polyester having an increased molecularweight.

MFR, Gel, and FE values obtained after a series of polymerization stepswas repeated ten times without disassembling and cleaning of thereaction apparatus are shown in Table 1.

Example 2

Pellets of an aliphatic polyester having an increased molecular weightwere obtained in the same manner as in Example 1 except that couplingreaction tank 8 without stirring blades was used and the reaction in thereaction tank was conducted without stirring.

Example 3

Pellets of an aliphatic polyester having an increased molecular weightwere obtained in the same manner as in Example 1 except that thealiphatic polyester prepolymer delivered from static mixer 6 was furtherdelivered to mixing tank 7 for agitation and stirring. Mixing tank 7 wasprovided with three partitioning plates and four stirring blades. Therotational speed of the stirring blades was 100 rpm. The time requiredfor the solution to be passed through mixing tank 7 was three minutes.

Example 4

Pellets of an aliphatic polyester having an increased molecular weightwere obtained in the same manner as in Example 3 except that couplingreaction tank 8 without stirring blades was used and the reaction in thereaction tank was conducted without stirring.

Comparative Example 1

A summary of the production steps of Comparative example 1 is shown inFIG. 2. Steps before the transesterification reaction were the same asin Example 1. In Comparative example 1, there was no static mixer 6 ormixing tank 7 and diisocyanate storage tank 5 was directly connected tocoupling reaction tank 8 via pipe 10 d.

In Comparative example 1, the esterification reaction step and thetransesterification reaction were completed as in Example 1. Theobtained aliphatic polyester prepolymer was delivered into couplingreaction tank 8 (delivery rate: 3,000 kg/hr, total amount: 6500 kg) inaccordance with the flow chart shown in FIG. 2. After completion ofdelivery, the predetermined amount (70.3 kg) of hexamethylenediisocyanate was injected from the upper portion (vapor phase) of thecoupling tank. The ingredients were agitated and stirred by helicalribbon blades, were maintained at 180° C. for 8 hours, and then werepelletized by pelletizer 9 to obtain an aliphatic polyester having anincreased molecular weight.

The diisocyanate measured was charged into diisocyanate storage tank 5and was delivered while being pressurized with nitrogen. (As it could bedelivered by adjusting the pressure of nitrogen, the diisocyanate couldbe quantitatively delivered without using a pump.)

MFR, Gelation, and FE values obtained after a series of thepolymerization steps was repeated ten times are shown in Table 1.

TABLE 1 Example 1 Example 2 Batch MFR Gel. Foreign MFR Gel. Foreign No.(g/10 min) FE matter (g/10 min) FE matter 1 1.4 1 A 1.5 1 A 2 1 1 A 1.81 A 3 1.6 1 A 1.2 1 A 4 1.2 1 A 0.8 1 A 5 1.8 1 A 1 1 A 6 0.9 1 B 0.9 1A 7 1.3 2 B 1.4 2 A 8 0.8 2 A 1.6 1 A 9 1 2 B 1.2 1 B 10 1.4 1 A 1.1 2 BAverage 1.24 1.25 S.D. 0.32 0.32

TABLE 2 Comparative Example 3 Example 4 example 1 MFR For- MFR For- MFRFor- Batch (g/10 Gel. eign (g/10 Gel. eign (g/10 Gel. eign No. min) FEmatter min) FE matter min) FE matter  1 1.3  1 A 0.8  1 A 3   1 A  20.9  1 A 1.3  1 A 2.2  1 A  3 1.2  1 A 1.5  1 A 2.8  1 A  4 0.8  1 B1.2  1 A 0.5  2 B  5 0.8  2 B 1   1 A 1.8  2 A  6 1.2  1 A 1.2  1 A 2.5 3 C  7 1.1  1 A 0.8  1 A 2.8  5 C  8 1.4  2 A 1.3  2 B 1.6  5 D  9 0.8 1 B 0.9  1 A 3.1  5 C 10 0.9  2 B 1.4  2 B 1.9  5 D Average 1.04 1.142.22 S.D. 0.23 0.25 0.80

The desired value of MFR is 1 g/10 min. The level of distribution ofvalues was less and the average value of 10 batches was relatively closeto the desired value in the examples, while the level of distributionwas significant and the average value was remarkably different from thedesired value in the Comparative example. In particular, it is indicatedthat the standard deviations were small and sufficiently controlled MFRswere obtained in Examples 3 and 4 using mixing tank 7.

Further, there is a tendency for the evaluation grades on Gel, FE, andforeign substance to become poorer as the batch number increased. Thisindicates that it was difficult to maintain the quality of the polymerwhen produced continuously.

EXPLANATION OF SYMBOLS

1: Weighing hopper; 2: glycol storage tank, 3: esterification tank, 4:transesterification reaction tank, 5: diisocyanate storage tank, 6:static mixer, 7: mixing tank, 7 a: inlet, 7 b: outlet, 7 c: stirringshaft, 7 d: partitioning plate, 7 e and 7 f: stirring blades, 7 g:nitrogen inlet, 8: coupling reaction tank, 9: pelletizer, 10 a-g: pipes,11 a-d: pumps

1. A method for producing an aliphatic polyester having an increased molecular weight comprising the steps of: (i) quantitatively injecting a diisocyanate into an aliphatic polyester prepolymer which has a number average molecular weight of 5000 or higher and has terminal hydroxyl groups and in which at least one acid component is a succinic acid compound, in a molten state at a temperature not lower than the melting point thereof, wherein the diisocyanate comprises isocyanate groups in an amount equivalent to between one tenth and two times the amount of the hydroxyl groups in the aliphatic polyester prepolymer; (ii) quantitatively and continuously mixing the diisocyanate-containing aliphatic polyester prepolymer obtained at step (i) by a static mixer, discharging same from the static mixer, and then supplying same to a coupling reaction tank, and (iii) reacting the aliphatic polyester prepolymer with the diisocyanate in the coupling reaction tank.
 2. The method for producing an aliphatic polyester having an increased molecular weight according to claim 1, wherein a mixing tank is further provided between the static mixer and the coupling reaction tank and the aliphatic polyester prepolymer of (ii) discharged from the static mixer is introduced into the mixing tank and is further discharged from the mixing tank under stirring to be supplied into the coupling reaction tank.
 3. The method for producing an aliphatic polyester having an increased molecular weight according to claim 2, wherein the mixing tank in (ii) has an inlet of the diisocyanate-containing aliphatic polyester prepolymer in its upper portion, an outlet for discharging a mixture of the aliphatic polyester prepolymer with the diisocyanate in its bottom portion, and at least one partitioning plate and stirring blade.
 4. The method for producing an aliphatic polyester having an increased molecular weight according to claim 3, wherein said at least one partitioning plate and stirring blade are spaced one above the other.
 5. The method for producing an aliphatic polyester having an increased molecular weight according to claim 3, wherein the stirring blade is a multistage blade.
 6. The method for producing an aliphatic polyester having an increased molecular weight according to claim 1, wherein the diisocyanate is injected by a non-pulsating pump at step (i) above.
 7. The method for producing an aliphatic polyester having an increased molecular weight according to claim 1, wherein the reaction at step (iii) above is conducted under stirring.
 8. The method for producing an aliphatic polyester having an increased molecular weight according to claim 7, wherein the reaction at step (iii) above is conducted under stirring by a helical ribbon blade or a twisted lattice-shape blade.
 9. The method for producing an aliphatic polyester having an increased molecular weight according to claim 1, wherein the reaction at step (iii) above is conducted without stirring.
 10. The method for producing an aliphatic polyester having an increased molecular weight according to claim 1, wherein at least one polyalcohol component in the aliphatic polyester is ethylene glycol.
 11. The method for producing an aliphatic polyester having an increased molecular weight according to claim 1, wherein at least one polyalcohol component in the aliphatic polyester is 1,4-butanediol. 